add file/image paste to the Claude composer via native clipboard

Flutter's clipboard is text-only and tmux/send-keys carry text only, so
a pasted file or image must reach Claude as an @path reference (per the
T-134 spike). A native clide/clipboard MethodChannel reads the non-text
clipboard: GTK (gtk_clipboard_wait_for_image/uris) on Linux, NSPasteboard
on macOS. The composer overrides PasteTextIntent — Ctrl/Cmd+V resolves a
file path or writes a clipboard image to a cache dir, inserts the @path,
and falls back to plain-text paste otherwise. No new package dependency.

macOS handler is written but unverified on this Linux box — needs a build
on a Mac. Linux path builds and is covered by tests.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

lib/builtin/claude/src/claude_pane.dart

@@ -8,6 +8,7 @@ import 'package:flutter/services.dart' show rootBundle;
 import 'package:flutter/widgets.dart';
 
 import 'claude_composer.dart';
+import 'clipboard_paste.dart';
 import 'conversation_controller.dart';
 import 'conversation_view.dart';
 import 'session_naming.dart';
@@ -268,6 +269,7 @@ class _ClaudePaneState extends State<ClaudePane> {
           ClaudeComposer(
             enabled: _paneId != null,
             onSubmit: _send,
+            pasteResolver: () => resolveClipboardAttachment(const NativeClipboard()),
           ),
         ],
       );